I'd not heard of chimera in cats until you mentioned it. I really don't know if she is or not. Based on reading, it seems the only way to know for sure is a DNA test. Are there other non-tests that would indicate such?
My only knowledge of chimerism is with people. I first heard of it watching CSI years ago, the story was of a set of twins, one of which turned out to be chimeric and almost got away with murder until they discovered his secret. I also saw a documentary on it, one of which was a woman who had applied for public assistance in the UK, and apparently they apply their due diligence over there. She had to have testing to prove that her children were hers, and the tests came back repeatedly that she wasn't their mother. I believe in the end they took a tissue sample of her uterus and that DNA apparently matched. There were a few other stories similar to this as well. I guess the idea was that there were twins (franternal) in utero and one was assimilated. I guess the assimilated tissue can then grow into organs and such.
Miss Muffin is such a pretty girl. I had a tortie Siamese and she was quite the pistol. I hope you bring each other years of joy together.